The catalysts used in thermal combustion systems for gas turbines provide low emissions and high combustion-efficiency. To achieve high turbine efficiency, a high gas temperature is required. To obtain such high temperatures, the catalyst temperature must be high, affecting the strength of the materials used for the catalyst structure and its supporting members. Hence, there is a need to provide a support for catalytic structures while maintaining the temperature of the support low enough so that its strength is not adversely affected by the high temperature of the catalytic process. This is especially advantageous for metallic catalyst structures and metal support members since the strength of metals decrease rapidly at temperatures above 700.degree.-800.degree. C.
In a catalytic combustion reactor for a gas turbine, high gas flow through the reactor and high temperature place very large stresses on the catalyst structure and reactor. This can result in cracking, fracture, or distortion of the catalyst structure and reactor during operation. Because of these adverse operating conditions, support structures can be used to support and retain the catalyst structure within the reactor.
A catalyst structure which may be used in such adverse conditions is a monolithic structure comprising a carrier of a high temperature resistant, relatively fragile material such as any ceramic or a metallic foil. Such a catalyst structure may be a honeycomb-like structure having a large number of thin-walled channels extending in the direction of the gas flow. The catalyst structure may be designed to accept support members.
The catalyst structure may be supported in a variety of ways, including structures placed at the outlet of the catalyst structure or circumferentially about the catalyst structure. All support structures are subject to the high temperature of the catalytic reaction, and often are cooled using externally induced cooling to maintain their strength. An example of a catalyst structure with a circumferential support is described in U.S. Pat. No. 4,432,207, to Davis Jr. et al. Davis Jr. et al. disclose a modular catalytic structure with support for the individual catalyst modules. The supports for the catalyst modules are circumferential sheet metal fabrications having integral passageways for cooling air. The proposed source of air is the gas turbine compressor. The disclosure is directed to a catalytic assembly made with catalytic sub-units to provide minimal stress due to thermal gradients. Davis et al. does not teach the use of a catalyst support using a structural component at the outlet of the catalyst to prevent axial movement of the catalyst.
Another example of a circumferential support is described in U.S. Pat. No. 4,413,470 to Scheihing et al. Scheihing et al. discloses a transition duct mounted catalytic element support for use in gas turbines. The catalytic element is supported on each end by a circumferential spring clip assembly, which also functions to hold the catalyst in position within the duct. This patent is directed toward a catalytic bed with a support system that can easily be retrofitted into existing gas turbines. Although the rear spring clip assembly is said to be capable of being cooled, Scheihing et al. is silent on a method of how to accomplish such a goal.
The use of a circumferential support in an application other than gas turbines can be found in U.S. Pat. No. 3,957,445 to Foster. Foster discloses an automotive emissions control catalyst design that uses a circumferential support that is spring loaded to maintain a good gas seal in and out of the catalyst. The spring and circumferential support are cooled by a pressurized air supply. The objective of this design is to provide good sealing for the gas flow into the catalyst independent of the thermal expansion of the catalyst and engine members.
U.S. Pat. No. 3,480,405 to Hatcher describes a structure to support a particulate or pelleted catalyst bed. The support consists of a complicated arrangement of plates, tubes, and internal passageways through which a cooling fluid is passed. This arrangement has the disadvantage of restricting the gas flow and causing a large pressure drop which would reduce the efficiency of the gas turbine. In addition, the size of this support structure would substantially cool the gas stream, a disadvantage in the case of the catalytic combustion process.
In those support structures which are cooled, air is often used as a cooling medium. However, other gases, or liquids can be used depending upon their availability and desirability. For example, in U.S. Pat. No. 3,480,405 to Hatcher, a liquid cooled support for a catalyst bed used in the production of HCN is disclosed. The Hatcher design also requires physical separation of the cooling medium from the reaction gas.
U.S. Pat. No. 5,026,273 to Conelison and the above discussed U.S. Pat. No. 4,413,470 to Scheihing show actual combustor designs for gas turbines. Neither of these designs show structures supporting the downstream face of the catalyst. Since these catalysts are typically quite large, 10 to 25 inches in diameter, the total force on the structure can be quite large. As an example, for a typical catalyst with a pressure drop of 3 psi due to the gas flow through the catalyst, the total force on the catalyst structure would be 240 lbs. for a 10 in. diameter catalyst and 1500 lbs. for a 25 in. diameter structure. To withstand this force, the catalyst structure would have to be quite long, have thick walls and be composed of materials with high strength. These are all disadvantages. Catalyst structures with several short sections could not be used in these designs. Also, materials with lower strength, such as metals operating at high temperature, could not be used as a catalyst support. In addition, cracking or distortion of the catalyst resulting in failure would allow part or all of the catalyst to travel into the power turbine blades causing severe damage and very costly repairs.
None of the documents discussed above suggest the internally-cooled support structure for securing a catalyst structure within a combustion reactor, as is described below.